We surveyed human hemotopoietic tumors and tumor cell lines for sequences capable of transforming NIH/3T3 cells by DNA transfection. We found that the transforming gene associated with one acute lymphocytic leukemia was the activated cellular homolog of K-ras, while N-ras was shown to be the activated form of several myeloid and lymphoid tumor cells. We have also detected an apparently new transforming gene in the DNA of a non-Hodgkin's lymphoma, characterized histopathologically as a diffuse undifferentiated B-cell lymphoma. It was demonstrated that the biologic activity of this transforming DNA was serially transmissible through several cycles of transfections, and that it can induce a series of phenotypic changes in the same cell. Moreover, this transforming gene is not related to any of the known ras genes, nor to several other retroviral and cellular oncogenes. By using the same NIH/3T3 cell transfection assay, we analyzed fibrosarcomas induced in C57B1/6 and NIH Swiss mice by subcutaneous single dose treatment of MCA. We used high molecular weight DNAs of the tumor cell lines derived from four of these fibrosarcomas for transfection on NIH/3T3 cells. We demonstrated that two of these tumor cell line DNAs are capable of transforming NIH/3T3 fibroblasts, that the transforming gene associated with these tumors is the same and that it is the cellular analog, K-ras, of the transforming gene of Kirsten murine sarcoma virus. We extended our studies to DNAs extracted from primary tumors developed in 75% of BALB/c and NIH Swiss mice treated SQ with a single dose of 100 Mug methylcholanthrene. High molecular weight was extracted from each of the tissue specimens and tested on NIH/3T3 by transfection assay. Fifty percent of the tumor DNA analyzed was able to transform the NIH/3T3 cells. We demonstrated that exogenous K-ras sequences were associated with the transformed phenotype of the transfectants and established that K-ras is the transforming gene activated at high frequency in MCA-induced fibrosarcomas.